The present invention relates to a thermal recording system for thermally recording a visible image on a recording sheet.
There have been invented and demonstrated a wide variety of thermal recording systems. One of them is a thermo-sensitive recording system wherein a heat-sensitive substance is heated to produce a color (See U.S. Pat. No. 3,539,375), and in another method known as a thermal ink transfer imaging process, a heat-sensitive coloring substance in the form of a carbon paper is overlapped over a plain paper sheet to transfer an image. (Reference is made to Japanese Patent Laid Open to Public Publication No. 51-15446). The recording sheet used in the thermo-sensitive recording process is a coated paper having a recording layer which seems similar to a plain paper but it produces a color forming no record when brought into contact with an adhesive containing an organic solvent. Furthermore finished copies remain sensitive to heat and can become increasingly dark so that the recorded images are lost. In addition, the color-fastness of the recorded images is so unsatisfactory that the recorded images will be lost within a year in a lighted room.
The thermal ink transfer imaging process is free from the defects of the thermo-sensitive recording process, but still has some defects. First, in addition to the recording sheets, carbon-paper-like coloring sheets must be prepared, and second the coloring sheets must be disposed of after recording.
In order to avoid these problems, there has been invented and demonstrated a recording system wherein the electrophotographic toner electrostatically or magnetically held on a recording sheet of plain paper is thermally fused and adhered to the recording sheet by a thermal print head and the unused toner is recovered for recirculation or reuse. (See French Pat. No. 80,814 which is a continuation of French Pat. No. 1,301,933). In this system, the thermal print head is brought into direct contact with the toner so that it is contaminated with the toner and consequently the recording sheet is contaminated with the toner. Furthermore, when the copying machine is stopped, the toner which has been adhered to the thermal print head adheres to the recording sheet, bridging between the recording sheet and the thermal print head.
In order to solve these problems, there has been invented and demonstrated a process wherein instead of a thermal print head, a laser beam is used to thermally adhere the toner to the recording sheet. (See Japanese Patent Publication Nos. 47-6160 to which U.S. Pat. No. 3,410,203 corresponds, and 47-15769 U.S. patent application Ser. No. 604,353 corresponds). However this process is economically disadvantageous in that it requires a relatively high power laser and a complex optical system for scanning the laser beam upon the recording sheet.